I've always found inconsistencies in high end with CDs (possibly differences in mastering technique between labels). MP3 compression of course doesn't help the matter, and the lower the bit rate the worse things can get. No real way to correct the sound of a bad or low bitrate MP3. But of course everything is perceptual, so if certain processing sounds better to most people, or sounds better in long or repeated listening sessions, chances are that the overall listening experience is improved.
Radio processors seem to help with this inconsistency by normalizing the spectral balance. Noise gates and expanders would definitely help with cleaning up quiet parts, but not sure if they'd help with shrillness per se. They will help with other aspects of bad sound quality though.
I've used the very beginning of Deep Purple - Smoke On The Water, to test and tune noise gate and expander settings. Seems that without noise gate / expander, but with agc / compress / limit, the noise at the beginning of the song between notes, gets boosted up like crazy. With properly tuned expander and/or noise gate, it would sound like my local radio stations when they play the song -- the notes are more defined, and the silence between the notes is actually silent.
Of course, an overly agressive noise gate or expander can ruin the sound by adding too much gain reduction to 'not quite silent' sounds.